1. Field of the Invention
The present invention relates to a measurement apparatus which uses low-invasive, alternating current testing means to survey and observe the status of an underground water leakage, the location, geometry, depth, and amount of underground streams and resources or of soil contaminants, and the progress of soil improvement. More specifically, the present invention relates to an underground exploration apparatus which measures at multiple points a voltage generated by conducting electric current through the ground to obtain measurement results at two or more frequencies, which uses an ground model that employs the finite element method or the boundary element method, an impedance network, or the like, which changes an unknown quantity, such as the electric conductivity, dielectric constant per frequency, or position information, of a local area of the ground model so as to make the error between the actual measured value and the calculated value smaller, to thereby identify a substance under the ground or detect a change in geological condition, and which displays input information and estimation processing results of the ground model two-dimensionally or three-dimensionally.
2. Related Background Art
Conventional techniques of underground exploration include, in addition to a boring method of digging up portions of the ground as an invasive method, non-invasive methods. Known non-invasive underground exploration methods are 1) an acoustic exploration method of causing a local earthquake by artificially-induced vibration, 2) an X-ray backscattering method of photographing light backscattered from a highly conductive substance by irradiation of soft X-rays, 3) a underground radar exploration method of utilizing microwave irradiation, 4) an infrared photography method of taking a photograph of the near-infrared region from the sky, 5) a millimeter wave monitoring method of monitoring scattering millimeter waves from the sky, and so forth.
The acoustic exploration method of causing a local earthquake by artificially-induced vibration utilizes a phenomenon in which approximately no oscillatory waves are reflected in a region where the geological condition is homogenous and accordingly a substance density is substantially identical throughout the region, resulting in uniform acoustic impedance, whereas oscillatory waves are reflected at the border between different geological conditions where a substance density is varied and the acoustic impedance is discontinuous. By catching oscillatory waves reflected by a plane where the geological condition is discontinuous, this method can measure with precision the thickness and depth of a geological layer three-dimensionally and can visualize the measurements to display if the acoustic velocity is known for each geological layer's condition. A drawback of acoustic exploration is that the method cannot find an unknown substance in a geological layer where the density is approximately uniform and no oscillatory waves are reflected. In short, in the acoustic exploration method, it is possible to provide the depth and thickness of a geological layer, but it is not possible to find what substances the geological layer is made of unless a proper drilling survey is conducted. For instance, the acoustic exploration method is incapable of detecting the depth of an underground stream and the presence of underground resources or measuring whether or not the soil is contaminated by chemicals.
X-ray backscattering is a recently established underground exploration method of photographing light backscattered from a highly conductive substance by irradiation of soft X-rays. This method utilizes the nature of soft X-rays which possess properties of electromagnetic waves and particles both and which exhibit, owing to the skin effect of electromagnetic waves, excellent permeability in substances that are low in electric conductivity and magnetic permeability, substances with high dielectric constant, or substances that contain light atomic nuclei while exhibiting low permeability to be scattered or reflected against substances that are high in electric conductivity and magnetic permeability, substances with low dielectric constant, or substances that contain heavy atomic nuclei. This method is suitable for checking the presence or absence of a shallowly buried object, about several tens cm from the ground surface. A drawback of the X-ray backscattering method is to explore only areas near the ground surface when the soil has high water content. Another drawback of this method is that it is incapable of identifying the geological condition.
The underground radar exploration method using microwave irradiation can explore only a relatively shallow depth since the skin effect of electromagnetic waves prevent electromagnetic waves from entering the soil of high water content and causes electromagnetic waves to attenuate. This method cannot identify the geological condition, either.
The infrared photography method of taking a photograph of the near-infrared region from the sky catches near-infrared light radiated from the ground and is therefor suitable for gathering information related to temperature, for example, for detecting a region in the ground that is higher or lower in temperature than the surrounding areas. However, this method is incapable of identifying the kind and depth of a substance.
The millimeter wave monitoring method of monitoring scattering of millimeter waves from the sky is an exploration method that utilizes the nature of iron-rich soil to absorb millimeter waves, which enter the atmosphere from the Van Allen Belt surrounding the earth as a result of solar activity and are scattered above the ground. A drawback of this exploration method is that it is applicable only to geological surveys near the ground surface.
An example of other non-invasive, or only slightly-invasive, underground exploration methods is a method of measuring electric potential or electric conductivity by conducting current through the ground.
Listed below are results of conducting Gazette Text Search on the Industrial Property Digital Library website of Japan Patent Office by inputting the search terms “[Scope of Claim=or (underground, geological condition, exploration)] AND [Scope of Claim=and (current, electrode)”.
The search found the following documents relating to techniques of detecting the location, point of corrosion, or site of coating damage of a buried pipe:
1) Japanese Patent Application Laid-Open No. 2002-071822, G01V 1/00, NOBUHIKO OYAMA, “POSITION DETECTING METHOD FOR BURIED PIPE”;
2) Japanese Patent Application Laid-Open No. 2003-004686, G01N 27/20, SHINNIPPON STEEL Corp., “METHOD FOR DETECTING DAMAGED LOCATION IN CORROSION-PROOF COATING OF EMBEDDED METAL PIPES USING INTEGRATING MEANS”;
3) Japanese Patent Application Laid-Open No. 2003-004687, G01N 27/20, SHINNIPPON STEEL Corp., “METHOD FOR DETECTING DAMAGED LOCATION IN CORROSION-PROOF COATING OF EMBEDDED METAL PIPES USING TWO KINDS OF FREQUENCY SIGNALS”;
4) Japanese Patent Application Laid-Open No. 2000-258280, G01M 3/18, FUJI ELECTRIC Corp., TAKESHI TAKASHIMA, “METHOD AND APPARATUS FOR DETECTING WATER LEAKAGE IN WATER SERVICE PIPE”;
5) Japanese Patent Application Laid-Open No. 2001-013111, G01N 27/82, NIOPPN KOUKAN Corp., “PAINT FILM DAMAGED POSITION DETECTING METHOD AND APPARATUS OF BURIED COATED STEEL”;
6) Japanese Patent Application Laid-Open No. 2000-019159, G01N 27/82, NIPPON KOUKAN Corp., “METHOD AND APPARATUS FOR DETECTING PAINT FILM DAMAGED POSITION OF BURIED STEEL PIPE”;
7) Japanese Patent Application Laid-Open No. 2000-019158, G01N 27/82, NIPPON KOUKAN Corp., “METHOD AND APPARATUS FOR DETECTING PAINT FILM DAMAGED POSITION OF BURIED STEEL PIPE”;
8) Japanese Patent Application Laid-Open No. 2000-088691, G01M 3/16, NISHIMATSU CONSTRUCTION Corp., “APPARATUS AND METHOD FOR MEASUREMENT OF LEAK OF WATER”; and
9) Japanese Patent Application Laid-Open No. H06-094568, G01M 3/40 7324-2G, MARUYAMA MANUFACTURER Co. Ltd., “METHOD AND APPARATUS FOR DETECTING LEAK OF WATER”.
Document 1) relates to a method of using two electrodes and indirectly measuring the conductivity of direct current while varying measurement points. This method utilizes the fact that there is a change in distribution of current conduction when two measurement points sandwich a buried pipe. A drawback of this method is that the result is easily influenced by water content, the diameter, direction, kind, and depth of the buried pipe, and other objects embedded in the ground. Documents 2), 3), 4), 5), 6), and 7) relate to a method of using a buried metal pipe as one of current application electrodes. Document 8) relates to a direct measurement method which utilizes the fact that a water leakage taking place on an array of pre-arranged electrodes causes a change in interelectrode impedance. Document 9) relates to an indirect measurement method which utilizes the fact that an electric resistivity between the soil portions of both side of a water channel is changed when the channel is closed and when the channel is opened.
The search found the following documents as prior art relating to geological surveys that use AC tests:
10) Japanese Patent Application Laid-Open No. H10-293181, G01V 3/06, OYO CHISHITSU Corp., “MULTICHANNEL ELECTRIC SURVEY METHOD WITH CAPACITOR METHOD”;
11) Japanese Patent Application Laid-Open No. 2001-074850, G01V 3/08, OYO CHISHITSU Corp., “ELECTRIC SURVEYING METHOD USING NON-POLARIZING ELECTRODE”;
12) Japanese Patent Application Laid-Open No. H09-127253, G01V 3/08, OYO CHISHITSU Corp., “MULTICHANNEL ELECTRIC SEARCHING SYSTEM EMPLOYING CAPACITOR METHOD”;
13) Japanese Patent Application Laid-Open No. 2002-156460, G01V 3/02, TLO KYUSHU Corp., LTD: KK “ELECTRIC SEARCHING METHOD, ELECTRIC SEARCHING APPARATUS USING THE SAME, AND LAND MINE DETECTING APPARATUS”;
14) Japanese Patent Application Laid-Open No. 2000-028743, G01V 3/12, MITSUI ENGINEERING & SHIPBUILD Corp., “BURIED OBJECT PROSPECTING METHOD AND NON-CONTACT HIGH-FREQUENCY CURRENT SENSOR FOR DETECTING CAVITY”; and
15) Japanese Patent Application Laid-Open No. H07-012766, G01N 27/00 D 9115-2J, KAJIMA CONSTRUCTION Corp., “UNDERGROUND FLOW CHARACTERISTIC MEASURING METHOD AND APPARATUS USING SPECIFIC RESISTANCE”.
Document 10) relates to a method of measuring direct current specific resistance with precision by employing a general AC test method to correct the influence of frequency and distance attenuation. Document 11) relates to a method of measuring the charging ratio and direct current specific resistance of electrodes by a general AC test method. Document 12) shows an embodiment of a system of measuring specific resistance by an AC test. Document 13) relates to an apparatus for detecting the presence or absence of a buried object immediately below a current electrode for measurement by measuring the electric potential of areas surrounding the current electrode for measurement while limiting current paths by providing an infinite distance current electrode and while changing the position of the current electrode for measurement. Document 14) relates to a method of measuring electric conductivity by an AC test. Document 15) relates to a method which uses an AC test together with tracer fluid injection.
The signal tracing method is included in the AC test method but uses a detector of the same resonance frequency as the applied frequency instead of measuring electric potential. In the search, the identified literature on the signal tracing method consisted of the following document:
16) Japanese Patent Application Laid-Open No. H06-130156, G01V 3/08 B 7256-2G, NIPPON TELEGRAPH & TELEPHONE Corp., “METHOD AND APPARATUS FOR SEARCHING EMBEDDED METAL”.
The following was found in the search as a document on an apparatus which uses a current path contrived to obtain specific resistance by numerical calculation and which displays a tomographic image:
17) Japanese Patent Application Laid-Open No. H05-100044, G01V 3/02 C 7256-2G, DIA CONSULTANT Corp., “AUTOMATIC ELECTRIC PROBING METHOD”.
Although the status of water leakage from a water service pipe buried under the ground can be detected even by development in the above prior art, those techniques cannot detect occurrence of the phenomenon called “sandblast” (a blast of soil and sand mixed with a water stream) which accompanies, for example, water leakage from a water service pipe buried in the vicinity of an embedded gas service pipe toward the gas service pipe, or a rapid water leakage.
In addition, the above prior art is capable of detecting how specific resistance (electric conductivity) is distributed but not of specifying the location, depth, and amount of underground streams and resources or soil contaminants, and substances contained in a geological layer, nor measuring or monitoring the progress of soil improvement since the electric conductivity is hardly changed by a change in frequency.
An object of the present invention is to provide a low-invasive technique of exploring under the ground using a low-invasive AC test method. More specifically, an object of the present invention is to provide a technical measure capable of specifying the location, depth, and amount of underground streams and resources or soil contaminants, and substances contained in a geological layer, or measuring or monitoring the progress of soil improvement.
Another object of the present invention is to provide a method of conducting an AC test without allowing AC noise, such as that caused by an electric current fed back or leaked to the ground from a power transmission line or from the overhead wire of a railway, to influence the test.
Still another object of the present invention is to detect the sandblast phenomenon caused by water leakage from a water service pipe that is buried in the vicinity of a gas service pipe.